Most efficient water temp settings on furnace

Hi there, thanks for this wonderful forum! I hope you can answer my question, because I've been searching everywhere online for answers and can't find any!

I am renting in a place with an old Weil McLain 68 Oil Boiler. It is the water heater as well as the heat for the house and it has no water storage tank, it is on-demand. We keep the place pretty cool around 60 degrees.
1. I've read that it is most efficient to keep the water temperature at the lowest setting (lo 120, hi 140), first is this true for on-demand heaters like mine that also heats the house? I also just read that you should NOT keep on-demand type heaters on the lowest setting since it's more energy to keep them at a low temperature.
Can you explain this?

Also what is the differential? Is it important to make sure it is the exact difference between the high and low?

Lastly, As I've said, I've been keeping the temperature settings on the lowest setting. With this setting I cannot get a hot shower. To get a shower I briefly turn the temperature gauge up to the highest setting and it seems to generally have enough hot water to have a 5-10 minute shower. Is this efficient? Is it most efficient to have it on the highest setting at all times for the type of water heater/heating system I have since there is no storage tank?

Thank you SO much if you can give me any insight to these questions that have been boggling my mind!!

For hot water production, you might find the best temperature is 190-170 or so. Maybe 200-180. When there is no tank, the water gets heated by flowing past the hot water in the boiler's tank reservoir. You get decent heat transfer only when there is a big temperature difference between them. The water flows by it for only a short time and distance, so if it isn't hot enough, it doesn't get hot. Think passing your hand through a candle flame. Do it fast and you might not even notice it's warm. Try the same thing with a blowtorch, though, and you'd notice the difference.

Older boilers don't like the cooler temperatures...you could get more condensation in the chimney, too, which could lead to other problems.

The water temperature won't affect the firing efficiency very much. It will have little effect on the combustion process because the heat exchanger is pretty much out of the firing zone.

Lower temperature on the water side results in more effective heat transfer because there is a greater temperature difference between the combustion products and the heat exchanger. Greater temperature difference will cause more heat to be extracted; therefore greater "efficiency" for that part of the process.

The greatest effect is probably on heat losses. Higher temperature settings for the water side will cause more heat to go up the stack because there will be greater heat transfer from the water to the air going up the stack when the boiler is not firing. Then when the water cools down it will cause the boiler to fire again just to keep it hot. The result is that you will be firing the boiler more to keep it hot without producing any result.

So all in all, lower boiler temperature setting will result in more heat transferred to the the house per gallon of fuel oil or therm of gas.

Now the down side. You need that higher temperature to get a good supply of hot water out of the tankless coil. That is especially a problem if the temperature of incoming water drops in the winter season.

I put an electric water heater in series with my tankless coil. It stores hot water and smooths out the temperature variations. It also heats the water if there should be a case where you have a long demand that the boiler/tankless coil can't keep up with. Because the boiler temperature is well above the temperature setting of the electric water heater the water heater rarely comes on during the winter heating season.

Another point about running a boiler at 120 is condensate. At any temp below 140 you will get condensation inside the boiler. tell your landlord they need to buy you an indirect fired hot water heater or they will need a new boiler and chimney in the next ten years